1. Field of the Invention
The present invention relates to an image scanning apparatus (an image scanning unit included in an image forming apparatus, such as a digital copying machine and a facsimile machine, or a stand-alone image scanning apparatus) such as an image scanner that reads an image from an original and an image forming apparatus that includes such as an image scanning apparatus.
2. Description of the Related Art
The above-described image scanning apparatuses include an image scanning unit and an A/D convertor (A/D converting unit). The image scanning unit reads an image from an original by scanning an image-formed surface of the original in the main-scanning direction with light that has been emitted from an illumination unit or a light source and converting, by using a photoelectric conversion element, the light reflected from the image-formed surface into electricity line by line, thereby creating an analog image signal. The A/D converter converts the analog image signal that has been created by the image scanning unit into a digital image signal and outputs the digital image signal.
A widely-known image scanning apparatus that operates in the above manner includes an image scanning unit that uses a contact image sensor (CIS) as the photoelectric conversion element for scanning. The CIS is, for example, includes a plurality of sensor IC chips (hereinafter, also referred to as “sensor ICs”, simply) arranged side by side and an equal magnification optical system.
A known image scanning apparatus (see, e.g., Japanese Patent Application Laid-open No. 2002-290686) includes a white member and an image scanning unit that uses a CIS for scanning. The white member faces the scanning surface of the image scanning unit. During a period other than the original sheet scanning time when the image scanning unit reads an image from the original, a time period, during which the image scanning unit scans the surface of the white member to create reference white data that is used for shading correction, is provided. There is an interval (gap) between the image scanning unit and the white member so that the original can pass through the interval.
A typical optical system that forms an image at the same magnification has a short optical path and a high concentration of light coming from the light source; therefore, if an original passes in loose contact, a large change occurs in the output level. Here, it is assumed that the amount of change in the output level caused by change in the interval (distance) between the image scanning unit and the original is defined as the depth-of-illumination property.
In an image scanning apparatus having a configuration in which the image scanning unit is fixed and scans an original being conveyed, it is necessary to make a gap that allows an original with a maximum sheet thickness to pass between the image scanning unit and the back. As the maximum sheet thickness increases, it is necessary to increase the gap.
In a typical image scanning apparatus, the white member, which is needed to create the reference white data, always faces the image scanning unit. Alternatively, in some image scanning apparatuses, the white member is configured to move to a position to face the image scanning unit when necessary.
In both cases, due to deviation of movement of the original, the interval between the image scanning unit and the white member and the interval between the image scanning unit and the original cannot always be equal and, in some cases, the intervals are not equal (because, even if the interval between the image scanning unit and the white member is constant, the interval between the image scanning unit and the original may vary). If the intervals are not equal, because of the influence of the depth of illumination, a problem that an undesired line is formed on the image may occur.
With reference to FIGS. 10 to 12, described is the reason why an undesired line is formed on the image because of the influence of the depth of illumination when the white member faces the image scanning unit.
FIG. 10 is a schematic diagram of a conventional image scanning apparatus in which an image scanning unit uses a CIS.
Light emitted from a light source 302 passes through a glass 305 and then illuminates a white member 301 or an original 306. The light reflected or diffused from the white member 301 or the original 306 is received, via a selfoc lens array (SLA) 303, by a sensor IC 304a that is on a sensor board 304. The received light is then converted into electricity and thus an analog image signal is generated.
The interval between the white member 301 to be scanned and the surface of the glass 305 when the white member 301 is scanned is A and the interval between the original 306 and the surface of the glass 305 when the original 306 is scanned is B; therefore, a position of the white member 301 when it is scanned and a position of the original 306 when it is scanned are different in terms of the height above the surface of the glass 305.
Because the scanned image contains unevenness due to the light source and unevenness in the sensitivity of the sensor, a typical image scanning apparatus needs shading correction.
The shading correction is performed using the following equation:Dout=(Din−Bk)/(Dsh−Bk)×(2n−1)    Dout: output image data after the shading correction    Din: image data obtained when scanning the original    Dsh: image data obtained when scanning the white member (shading data)    Bk: black level (level of image data obtained with no light)
FIGS. 11 and 12 are graphs that illustrate a problem that can occur when an image of the white member 301 and an image of the original 306 shown in FIG. 10 are scanned.
In these graphs, the distribution in the main-scanning direction of image data obtained when the white member 301 is scanned is shown as the shading data, and the distribution in the main-scanning direction of image data obtained when the original 306 is scanned is shown as original sheet data (image data of the original).
When Dout is calculated using the above equation with the shading data being substituted as Dsh and the original sheet data being substituted as Din, as shown in (a) of FIG. 11, if the ratio between the output of Dsh and the output of Din is the same at every pixel position in the main-scanning direction, as shown in (b) of FIG. 11, flat image data is output after the shading correction. In this time, it is assumed that the original scanned in the above example has an even density.
If the depth-of-illumination is uneven along the main-scanning direction, the following problem occurs.
The graph (a) of FIG. 12 illustrates an example where the depth-of-illumination has unevenness near the center in the main-scanning direction. It is clear from the graph that the difference between the scanning level at the interval A and the scanning level at the interval B shown in FIG. 10 is different between at the center section and at the sections other than the center section.
In this case, even when an image on the original having an even density is scanned, as shown in (b) of FIG. 12, the image data output after the shading correction is not flat.
The not-flat part in the image data appears repeatedly in the sub-scanning direction at positions aligned in the sub-scanning direction, and thus an undesired vertical line is formed on the image.